How to rotate multiple (controller) bones with one main (controller) bone?
Here's my version of the file. This is what I did to get the desired functionality:


Go into Edit Mode, select all the bones which make up the finger/feather controls only, and press Alt + p to bring up the Unparent menu. Choose Clear Parent.
Go into Pose Mode, then select the Bone Constraint Properties tab in the Properties Panel. Access the Bone Constraint pulldown from the top of the panel, and create a Child Of constraint. Set the Target to Armature, and the Bone to SmallWingCurve.L. Finally, Press the Set Inverse button to ensure that the new parenting relationship preserves the child's current position.
Copy new Child Of constraints to all of the remaining Small_Feather_Controller bones. Select all of these except the one to which you just added the Child Of constraint. Then, hold Shift and add the one which has this constraint to the selection. It must be the last one added to the selection, as this makes it the active object. Then, from the Pose menu at the top of your viewport, select the Constraints submenu, then Copy Constraints to Selected Bones. Now all your feathers/fingers have the same constraint.


If you try to rotate your master controller now, all the smaller ones rotate with it.

One advantage of the Child Of constraint is that it allows you to keyframe changes to the parenting relationship. This is not possible with direct parenting. Another is that the resulting parenting relationship can be increased or decreased by degrees using a slider, rather than being a binary on-off relationship only. Yet another is that more than one parent can be assigned to a child object in this way, whereas this too is not possible via direct parenting. Perhaps the most important advantage is that it seems to work for your purposes here, where direct parenting did not. I'm not entirely clear as to why. If anyone knows the answer, please explain this for me.